why do animals not affected by aids?
For the most part viruses and bacteria tend to infect only one particular species, giving them light to moderate symptoms.
When two species are in close, frequent contact it allows viruses to more easily jump into others.
The black plague, for example, is pretty much harmless to rats, and also harmless to the fleas that drink the rats’ blood.
But when the infected flea bites a human, the disease basically goes nuts.
If a bacterial infection or virus is too deadly it basically kills itself by destroying the pool of potential hosts.
So generally an infection has a “original” host that it’s either nonsymptomic or somewhat annoying to deal with rather than deadly, or it has as broad a selection of hosts as possible to get around it killing off its hosts (like Rabies).
The most easily spread viruses are ones that are spread via respiration/aerosol, this is where you have your bird flu and swine flu and the like.
HIV isn’t respiratory, it’s limited mostly to blood and sexual secretions.
As for the most part humans don’t go around banging animals, that means the potential transmission for HIV to an animal requires the human to be bleeding and the animal to also be bleeding.
There aren’t a lot of circumstances where that happens, because if an animal is causing a human to bleed, that animal is going to get itself shot dead.
So HIV doesn’t really have the opportunity to spread into an animal population, even if it could survive in said animal population.
But there are HIV-like diseases present in various species of animals, monkeys and apes have Simian Immunodeficiency Virus and cats have Feline Immunodeficiency Virus.